1. Field of the Invention
The invention relates to a flexible printed circuit assembly, and in particular, to a flexible multilayer printed circuit assembly that effectively reduces dielectric loss and decreases overall construct thickness by using a fluorocarbon dielectric layer, and an adhesive layer after lamination that is thinner than a signal trace.
2. Background Information
As processing speeds and packaging densities have increased, traditional wire cables cannot meet the requirements of high-speed signal interconnections. Further, high-performance coax or optical fiber has drawbacks, such as, poor I/O density, a greater overall cost, lack of decoupling and DC distribution, and a greater consumption of precious physical space. Thus, flexible printed circuit assemblies have been developed for the high-speed interconnections between multiple computer processing units. These flexible printed circuit assemblies are high circuit density systems that can save space, can handle multiple conductive layers with their high circuit density, and can provide high-speed interconnections between multiple units. Because its flexibility is greater than a rigid printed circuit assembly, the flexible printed circuit assembly may be used where the assembly needs to be bent to interconnect computer units together. These benefits have increased the use of the flexible printed circuit assemblies in place of the traditional wire cables for computer processor interconnections.
A typical flexible printed circuit assembly has multiple conductive layers that are made of conductive material. Each conductive layer is separated by a dielectric layer. Generally, the conductive layers include ground layers used to provide a reference voltage plane or a logic ground plane. The flexible printed circuit further includes a signal trace made of conductive material, which is used to provide a high-speed signal interconnect, and an adhesive layer encompassing the signal trace.
FIG. 1 illustrates a conventional flexible printed circuit assembly 10 is provided. The flexible printed circuit assembly 10 includes a bottom copper ground layer 11, a first dielectric layer 12, a signal trace 13, a first adhesive layer 14, a second dielectric layer 15, a second adhesive layer 16 and a top copper ground layer 17. Both the signal trace 13 and the first adhesive layer 14 are disposed on the top surface of the first dielectric layer 12. The adhesive layer 14 bonds the second dielectric layer 15 to the first dielectric layer 12.
As shown in FIG. 1, the adhesive layer 14 is typically thicker than the signal trace 13, with the signal trace 13 being encased within the adhesive layer 14. Accordingly, the adhesive layer 14 fully encompasses the signal trace 13, and surrounds the entire side surface of the signal trace 13, also covering a top surface of the signal trace 13. While this structure of the adhesive layer 14 provides a manufacturable and reliable flexible printed circuit assembly 10, the adhesive layer 14 is typically made of a bonding material with a relatively large dielectric loss tangent, which attenuates energy from the propagating signal from the signal trace 13. The bonding material may be epoxy resin, acrylic, silicone, polyurethane, or any other adhesive material.
Dielectric loss tangent is a constant that determines the “lossiness” of a certain material. A low dielectric loss tangent results in a low loss material, allowing a high-speed interconnect, while a large dielectric loss tangent results in a high loss material or a significant attenuation material, allowing a relatively low-speed interconnect. The amount of dielectric loss may be determined by the amount of material surrounding the signal trace 13, and the dielectric loss tangent of the surrounding material. In this context, because the adhesive layer 14 entirely covers the signal trace 13, the most significant dielectric loss contributor for the signal trace 13 is the adhesive layer 14, i.e., the thickness of the adhesive layer 14 and the magnitude of its dielectric loss tangent. Because of the relatively large dielectric loss tangent and thickness of the adhesive layer 14, the flexible printed circuit assembly 10 may not be suited for high-speed applications. Further, due to the thick adhesive layer 14, the flexible printed circuit assembly 10 has an increased overall construct thickness, bringing about less mechanical flexibility.
Further, both the first and second dielectric layers 12 and 15 are typically made of the same material, such as Kapton™, polyimide, or any other dielectric material, which makes it difficult to enhance the dielectric loss characteristics of the flexible printed circuit assembly 10.